Explanation: On May 9,
the diminutive disk of Mercury spent about seven and
a half hours crossing in front of the Sun
as viewed from the general
vicinity of Earth.
It was the second of 14 transits of the Solar System's innermost
planet in the
21st century.
Captured from Fulham, London, England, planet Earth the tiny
silhouette shares
the enormous solar disk with
prominences, filaments, and active regions in this sharp image.
But Mercury's round disk (left of center)
appears to be the only dark spot,
despite the
planet-sized sunspots
scattered across the Sun.
Made with an H-alpha filter that narrowly transmits the red
light from hydrogen atoms, the image emphasizes
the
chromosphere,
stretching above the photosphere or normally visible solar surface.
In H-alpha pictures of the chromosphere, normally dark sunspot regions
are dominated by bright splotches
called
plages.